* Fissionable: This means a material can undergo nuclear fission when struck by a neutron.
* Sustaining a Nuclear Reaction: A nuclear chain reaction requires a certain amount of the fissionable material (called the critical mass) and the right conditions to keep the reaction going.
So, if a material is fissionable, it *could* potentially sustain a reaction under the right conditions. It's just a matter of whether those conditions are met.
Here's what's likely happening in your question:
1. You might be thinking of materials that are difficult to make fission. Some isotopes are less likely to undergo fission than others. For example, Uranium-238 is much harder to split than Uranium-235. This doesn't mean it can't fission, it just means it requires more energy and is less likely to do so spontaneously.
2. You might be thinking of materials that are not suitable for a reactor. Some fissionable materials are very radioactive or have short half-lives, making them unsuitable for use in a nuclear reactor.
Examples:
* Uranium-238: While it can undergo fission, it's not the primary fuel used in nuclear reactors because it's less likely to fission and needs more energy. It plays a role in the breeder reactor concept, though.
* Plutonium-239: This is a key fuel in some reactors, but it's very radioactive and has a shorter half-life than Uranium-235. This makes it more difficult to handle and store safely.
To summarize: Fissionable materials can always sustain a nuclear reaction *if* the right conditions are met. We might choose not to use some materials due to their properties, but that doesn't mean they're fundamentally incapable of sustaining a nuclear reaction.
